Unsurprisingly, this rocks. It's a juvenile-market novel, and starts off with Our Heroine remembering her mother's pirate past and escaping civilised society for a life of adventure. There's a nice twist a third of the way in, and then things kick into gear somewhat.
It suffers from the same thing all pirate stories suffer from: pirates, frankly, are nasty people who do terrible things, but this a hero does not make. So instead, there are many, many hoops to leap through in order to have the romantic image of a pirate without, y'know, the piracy.
Plus, there's some extremely far-fetched puzzle-solving. I haven't read any Dan Brown, but I get the impression this is of the same order.
But this is a YA novel, so what the hell. There's a touch of the superhero here, but it's all good fun, and you just know you can't go wrong with Tanith Lee.